My black neon tetra has medium size spots on his body what is it

What is the water volume of the tank? 84litre or 20gallons
How long has the tank been running? 28th of may
Does it have a filter? Yes
Does it have a heater? Yes
What is the water temperature?i try to keep it at 23 but it has become very hot resantly outside so the temp is now at 27
What is the entire stocking of this tank?
2 black neon tetras
1 cardanal tetra
1neon tetra
1 male dwarf gourami
2 female dwarf gouramis
And 3 guppies

Maintenance
How often do you change the water? I have done it once as its a new aquarium
How much of the water do you change? 20 percent
What do you use to treat your water? Idk
Do you vacuum the substrate or just the water? Both

*Parameters - Very Important
Did you cycle your tank before adding fish? Yes
What do you use to test the water? Nothing
What are your parameters? We need to know the exact numbers, not just “fine” or “safe”.

Ammonia:
Nitrite:idk
Nitrate:idk
pH:idk

Feeding
How often do you feed your fish?daily
How much do you feed your fish? A pinch
What brand of food do you feed your fish?tetra mine flakes
Do you feed frozen or freeze-dried foods?frozen and flakes

Illness & Symptoms
How long have you had this fish?3weeks
How long ago did you first notice these symptoms? Last night
In a few words, can you explain the symptoms?white spots medium sized near but not on the dorsal and tail fin
Have you started any treatment for the illness?
Was your fish physically ill or injured upon purchase?
How has its behavior and appearance changed, if at all?

Explain your emergency situation in detail. (Please give a clear explanation of what is going on, include details from the beginning of the illness leading up to now)gy

 

Do you happen to have a picture?

Are the fish flashing on rocks or decor? Are the white spots raised, looking similar to salt?

 

I’m worried about a couple of things-

your stocking numbers are very low-Tetras need to be in groups of 6 for each species

You’ve only changed the water once in a month, this should be a weekly occurrence

You don’t seem to treat your water with dechlorinator, and this can be an issue if you use tap water

I’m also worried if you have/know how to cycle an aquarium if you have nothing to test it with and no test results from before

I think you’re describing ich, but I’m not sure as I’ve not had to personally deal with it so I’ll let someone else chime in on that.

Buy a bottle of SeaChem Prime, an API Freshwater Test Kit and then we can start to dissect more.

 

These are generic suggestion but not relevant to the op specific issue.

Picture would be helpful but I tend to agree with LadfromLondon that it sounds like ick. The normal treatment recommend for ick (see sticky here) is heat.

ICH (ICK) Life Cycle and Natural Treatment | 153206 | Freshwater Fish Disease and Fish Health

For the heat treatment to be successful the entire tank has to raise to around 88 (not just the position of the thermometer). Also raising the heat should be done very slowly - two or three degrees per 24 hours. You should probably raise it to around 88.
-
However - before treating you should confirm it is ick. Also healthy fishes will often resist ick so if only one fish has ick it might be safer to just dispose of it; though most of your fishes should be able to handle the heat with perhaps the neon having the most difficulty.

I’m worried about a couple of things-

your stocking numbers are very low-Tetras need to be in groups of 6 for each species

You’ve only changed the water once in a month, this should be a weekly occurrence

You don’t seem to treat your water with dechlorinator, and this can be an issue if you use tap water

I’m also worried if you have/know how to cycle an aquarium if you have nothing to test it with and no test results from before

I think you’re describing ich, but I’m not sure as I’ve not had to personally deal with it so I’ll let someone else chime in on that.

Buy a bottle of SeaChem Prime, an API Freshwater Test Kit and then we can start to dissect more.

 

I agree with ladfromladon you need to do those things.